Almost every country in the world uses the metric system as its official system of measurement. The United States is the largest holdout, still using imperial (technically "US customary") units for most everyday purposes — distances in miles, weight in pounds, temperature in Fahrenheit — even though metric is standard in US science, medicine, and the military.
The United Kingdom sits in between: it officially adopted metric decades ago, but imperial units like miles (for road distances), pints (for beer), and stone (for body weight) remain common in daily life.
| Imperial | Metric equivalent |
|---|---|
| 1 inch | 2.54 cm |
| 1 foot (12 in) | 30.48 cm |
| 1 yard (3 ft) | 0.9144 m |
| 1 mile (5,280 ft) | 1.609 km |
| Imperial | Metric equivalent |
|---|---|
| 1 ounce | 28.35 g |
| 1 pound (16 oz) | 0.4536 kg |
| 1 stone (14 lb) | 6.35 kg |
| Imperial (US) | Metric equivalent |
|---|---|
| 1 fluid ounce | 29.57 mL |
| 1 cup | 0.24 L |
| 1 pint (US) | 0.473 L |
| 1 quart (US) | 0.946 L |
| 1 gallon (US) | 3.785 L |
Worth noting: UK (imperial) gallons and pints are larger than US ones — a UK gallon is about 4.55 liters, versus 3.785 liters for a US gallon. When a recipe or spec doesn't say which one it means, US measurements are the safer default assumption unless you know the source is British.
Metric uses Celsius, where water freezes at 0° and boils at 100°. Imperial uses Fahrenheit, where those same points are 32° and 212°. See the dedicated Celsius to Fahrenheit guide for the full formula and reference table.
Imperial units grew out of centuries of informal, regional measurement — a "foot" was originally based on the length of an actual human foot, and units varied by trade and location before being standardized. Metric was designed from scratch in the late 18th century specifically to be logical and consistent: every unit relates to the next by a power of ten, so converting between them (say, centimeters to meters to kilometers) is just moving a decimal point.
That consistency is the main practical advantage of metric — no memorizing that there are 16 ounces in a pound but 2,000 pounds in a ton, or 3 feet in a yard but 1,760 yards in a mile. It's also why metric is the standard for science and international trade, where consistency and easy unit conversion matter more than cultural familiarity.
For any specific value — not just the common reference points above — use the Imperial ⇄ Metric category on the UnitDial converter. It covers length, weight, volume, and temperature, and filters the unit list automatically so you only see valid pairs.